Roadway repair is often accomplished by overlaying the existing pavement (whether of concrete or asphalt paving material) with a new layer (often called a leveling course) of concrete or asphalt paving material. Without prior surface treatment, however, this method of repair generally results in the application of insufficient quantities of paving material in the rutted, pot-holed or otherwise damaged areas, because the overlay will be applied at the same rate per unit of roadway width in damaged areas (which have a greater depth to be filled across the width) as in the undamaged areas. The resulting reduced thickness in the overlay of the previously damaged areas will lead to renewed rutting or other wear damage in the new pavement in relatively short order. However, by milling the surface of the damaged pavement to a uniform surface elevation below the level of the damage, the addition of new pavement will produce a road surface having a consistent elevation across the entire width of the roadway. This repaving technique can be used to return the elevation of a damaged roadway to its original pre-damaged elevation, whereas the placement of a leveling course atop damaged but un-milled pavement will tend to raise the surface of the roadway or some portion thereof above its original elevation. Roadway repair without milling can require the raising of road shoulders, guardrails and manhole covers and the adjustment of overpass clearances, all of which are unnecessary if a proper milling technique is employed. A use of milling prior to repaving can also pert lit ready establishment of the proper road grade and slope, and thereby avoid drainage and safety problems. Furthermore, milling typically provides a rough surface that readily accepts and bonds with the new asphalt or other pavement overlay. Finally, milling can provide raw material that can be reclaimed for use in the production of new paving materials.
A milling machine typically comprises a wheel-driven or track-driven vehicle that includes a milling drum having a plurality of cutting teeth disposed around its periphery. The milling drum is mounted for rotation about a substantially-horizontal axis within a drum housing on the frame of the machine. Steerable wheel-drive or track-drive assemblies operated by hydraulic motors are provided to drive the machine in a milling direction and to steer it along a desired milling path. The drive assemblies are attached to lifting columns that include internal linear actuators which can be activated to raise and lower the frame of the machine with respect to the roadway surface. Wheel-driven machines include four ground-engaging wheel-drive assemblies, one at the left front, one at the right front, one at the left rear and one at the right rear. Track-driven machines include three or four ground-engaging track-drive assemblies including one at the left front and one at the right front. Some such machines will also include a third track-drive assembly at the left rear and a fourth at the right rear; however, some track-drive machines will have only a single, center-mounted rear track-drive assembly.
Since the milling drum is mounted for rotation in a housing on the frame of the machine, raising the frame on the lifting columns can raise the milling drum out of contact with the roadway surface, and lowering the frame on the lifting columns can lower the milling drum into contact with the road surface so as to make a cut of the desired depth. The milling drum is rotated by a primary drum drive assembly typically comprising a drive belt driven by a diesel engine, which drive belt engages a drivetrain comprising a sheave on an input drive shaft for the milling drum. A gear box is typically located between the sheave and the milling drum and includes a gear train and an output drive shaft on which the milling drum is rotated. The gear box thus allows for rotation of the output drive shaft for the milling drum at a speed and torque that is different from that of the input drive shaft. A milling machine may include a conveyor system that is designed to carry the milled material that has been cut from the roadway by the rotating milling drum to a location in front of, to the rear of, or beside the machine for deposit into a truck for removal from the milling site. Power for operation of the hydraulic motors that are typically employed to operate the conveyors and the drive assemblies is usually provided by the diesel engine.
A road stabilizer is a type of milling machine that does not include a conveyor system which is designed to carry the milled material that has been cut from the roadway by the rotating milling drum away from the machine. Instead, the milling drum of a road stabilizer is generally employed to mill or pulverized an existing road bed or roadway to a greater depth than does a milling machine prior to repaving (usually called reclaiming) or prior to initial paving (usually called stabilizing), and it leaves the pulverized material in place. The pulverized material left behind is usually compacted and covered with one or more additional layers of crushed aggregate material before paving.
Cold in-place recycling (“CIR”) machines can be used to repair damage to a roadway in a single pass, while reusing essentially all of the existing asphalt paving material. In the CIR process, damaged layers of asphalt pavement are removed. The removed material is processed and replaced on the roadway and then compacted. If a roadway has good structural strength, CIR can be an effective treatment for all types of cracking, ruts and holes in asphalt pavement. CIR can be used to repair asphalt roadways damaged by fatigue (alligator) cracking, bleeding (of excess asphalt cement), block cracking, corrugation and shoving, joint reflective cracking, longitudinal cracking, patching, polished aggregate, potholes, raveling, rutting, slippage cracking, stripping and transverse (thermal) cracking. CIR can almost always be used when there is no damage to the base of the roadway. Generally, CIR is only half as expensive as a new pavement overlay while providing approximately 80% of the strength of new pavement. CIR can be carried out by a CIR machine comprising a milling machine or a road stabilizer machine that has been modified by mounting an additive spray bar in the milling drum housing to inject an asphalt emulsion or foamed asphalt cement additive into the milling drum housing. The asphalt emulsion or foamed asphalt cement additive is then thoroughly blended with the milled material by the milling drum and can be left in a windrow or fed by the CIR machine's discharge conveyor directly into an asphalt paving machine. When a CIR process is carried out by a modified milling machine or road stabilizer, the additive material is supplied from a separate additive supply tank truck that is coupled to the modified milling machine or road stabilizer machine. The additive material is drawn directly from the tank on the additive supply truck and metered through an additive flow system that is mounted on the milling machine to the spray bar in the milling drum housing.
Because the milling drums of a milling machine and a road stabilizer (including those modified to perform a CIR process) operate in the same way for purposes of this invention, the term “milling machine” will be used hereinafter as a generic term that describes all of these machines.
During the operation of a milling machine, the lifting columns are employed to set the depth of the cut of the milling drum, and the machine operator advances the milling machine at a rate that permits the milling drum to make the desired cut in the roadway. However, circumstances may arise in which the milling drum encounters increased density material in the roadway during a milling operation, causing the machine to rise up out of the cut. In such circumstances, a milling machine having an automatic grade control system will attempt to compensate for the rise in the milling drum by lowering the frame on the lifting columns. This will cause an undesirable portion of the weight of the milling machine to be supported by the milling drum instead of the lifting columns, and may result in a loss of steering or braking control because of insufficient contact between the drive assemblies on the lifting columns and the roadway. This occurrence is dangerous for the machine operator and may also lead to damage of the milling machine. For example, if the reaction forces exerted by the roadway surface on the milling drum exceed the opposing forces applied to the milling drum by the lifting columns, the machine may lurch backward or forward depending on the direction of rotation of the milling drum. If the milling machine is operating in a down cut mode (i.e., with the milling drum rotating so as to cut downwardly in the direction of travel of the machine), the machine may lurch forward, whereas if the machine is operating in an up cut mode (i.e., with the milling drum rotating so as to cut upwardly in the direction of travel of the machine), the machine may lurch backwards. The terms “kick-back”, “kick-back event”, and similar terms will be used hereinafter to describe the lurching, either forward or backward, that occurs or may occur when the milling drum encounters conditions that cause an undesirable portion of the weight of the milling machine to be supported by the milling drum instead of the lifting columns.
U.S. Pat. No. 5,318,378 describes a milling machine having a kick-back control system that includes a load cell or strain gauge that is attached to the inner leg tube of a lifting column. This load cell or strain gauge is adapted to sense the compressive force, transmitted through the inner leg tube, which is imparted to the frame of the milling machine by the kick-back event. The load cell or strain gauge is operatively connected to a controller that may be activated to raise the frame on the affected lifting column and/or to stop the rotation of the milling drum. However, a disadvantage of this kick-back control system is that it can only react after a kick-back event has occurred. Another disadvantage of this kick-back control system is that location of the load cell or strain gauge on the inner leg tube of a lifting column places the load cell or strain gauge on a component (i.e., the inner leg tube) that moves with respect to the frame during normal operation of the milling machine. This subjects the load cell or strain gauge to bending moments caused by the imposition of directional forces due to normal forward motion or steering of the drive assembly. These bending moments could affect the ability of the load cell or strain gauge to accurately sense the compressive forces caused by a kick-back event and could lead to a failure to react to a kick-back event, or to a kick-back reaction when no kick-back event has occurred.
U.S. Pat. No. 5,879,056 describes a milling machine having a separate wheel assembly that is adapted to rotate in a forward direction when the machine travels in a forward direction and in a backward direction when the machine travels in the opposite direction. The assembly also includes an electronic sensor that detects when the wheel travels in the backward direction and signals a controller to disable the milling drum when the wheel travels in the backward direction by a predetermined distance.
U.S. Pat. Nos. 8,128,177, 8,292,371 and 8,632,132 describe milling machines that include strain gauges that are mounted on opposite side walls of the milling drum housing and are adapted to measure the reaction force of the roadway surface on the milling drum that is transmitted through the drum housing. These strain gauges transmit signals to a controller when the reaction force has exceeded a predetermined level, and the controller then reduces the power to the machine drive assemblies and/or reduces the rate at which the milling drum is lowered into contact with the roadway surface.